Typically, snowmobiles have two front skis connected to a handlebar and a rear endless track connected to an engine to propel the snowmobile. In such typical snowmobiles, the snowmobile is steered by turning the handlebar in the direction in which the driver wants to turn. This is known as steer-in-direction. During a turn, the suspension assembly connected to the ski on the outside of the turn is compressed, causing the snowmobile to roll slightly toward the outside of the turn, which the driver can compensate by leaning toward the inside of the turn.
In other vehicles, such as motorcycles, the vehicle is steered by what is known by countersteering. In a motorcycle for example, the motorcycle turns by leaning in the direction of the turn. To achieve this leaning, the driver momentarily turns the handlebar in the direction opposite to the direction of the turn (i.e. the driver countersteers).
Driving a vehicle that can lean into a turn offers a much different driving experience than driving a vehicle that is steer-in-direction. It would be desirable to at least some drivers to have the driving experience of a leaning vehicle on a vehicle having three ground engaging members such as a snowmobile.
U.S. Pat. No. 7,648,148 B1, issued Jan. 19, 2010, and United States Patent Publication No. 2007/0176384 A1, published Aug. 2, 2007, the entirety of both of which is incorporated herein by reference, describe three-wheel vehicles that can lean in a turn. In vehicles of this type, the frame of the vehicle is pivotally connected to a shock tower (also known as a transfer frame) and the front shock absorbers associated with the two front wheels are connected at their upper end to the shock tower. During a turn, the frame pivots into the turn like on a motorcycle, but the shock tower remains essentially vertical. As a result, the front shock absorbers do not get compressed due to the leaning of the frame.
Leaning of such vehicles having three ground engaging member can be achieved in two ways. The first is assisted leaning in which an actuator actively pivots the frame relative to the shock tower based on signals received from one or more sensors on the vehicle, some examples of which are provided in U.S. Pat. No. 7,648,148 B1. The second is unassisted leaning, in which the pivoting of the frame relative to the shock tower is the result of the countersteering effected by the driver and the balancing of the forces acting on the vehicle during the turn.
One of the advantages of unassisted leaning over assisted leaning is that a vehicle with unassisted leaning is mechanically and electronically less complex than an assisted leaning vehicle as it does not require an actuator, its associated components, sensors and an electronic control system to control the actuator based on the signals from the sensors. However, one of the advantages of assisted leaning over unassisted leaning is that the actuator can be used to assist to initiate leaning of the frame relative to the shock tower and to return the frame in the upright position.
Also, unassisted leaning vehicles rely on the friction between the ground and the ground engaging members in order to achieve leaning of the vehicle and for returning the vehicle to the upright position. Therefore, when the vehicle operates on a low friction ground surface, such as ice or snow, the friction between the ground and the ground engaging member may be insufficient to prevent the ground engaging members from sliding sideways which may lead to the vehicle tipping over or understeering.
As such, there is a need for a system for complementing the torques generated when countersteering in order to lean a frame of an unassisted leaning vehicle having at least three ground engaging members and to return the frame to an upright position.